The present invention relates to a non-adhesive photocuring sheet capable of giving molded articles with excellent outer appearance, design properties, mar-proof properties, chemical resistance and weather resistance, and to molded articles and a process for their manufacture. The photocuring sheet is particularly useful as an automobile interior material for instrument panels, console boxes, meter covers, door lock bezels, steering wheels and the like, as an automobile exterior material for weather-strips, bumpers, strut mounts, side molds, door molds, window molds and the like, as a material for front panels and buttons for AV devices and household appliances, and as an exterior furniture material or a building and house interior and exterior material.
As methods for shaping plastic articles while providing decorated surfaces there have been proposed (1) a method in which a pattern is preformed in the die surface, (2) a method in which a transfer film is fitted on the inside of the die and the pattern of the film is transferred to the outside of the molded article during molding, and (3) a method in which a functionalized or printed sheet is attached to the inside of the die and the sheet is attached to the molded article surface during molding. Methods (2) and (3) include the methods described in Japanese Unexamined Patent Publication No. 60-250925, Japanese Examined Patent Publication No. 59-36841 and Japanese Examined Patent Publication No. 8-2550, whereby a weather-resistant sheet or printed sheet is formed on the inside of the die, and a molding resin is used for injection molding to produce a molded article whose surface is covered with the sheet.
However, because this technique is carried out with transfer of thermoplastic sheets or printing to provide decoration and functionality, the surface hardness of the resulting molded articles is insufficient. For example, a highly weather resistant sheet made of polyvinylidene fluoride (PVDF) or the like may be used to give the molded article weather resistance, but its problem is a lack of sufficient surface hardness. To counter this, pre-crosslinked high-surface-hardness sheets must be used in order to obtain molded articles with high surface hardness. Such sheets, however, cannot be easily employed for molded articles with three-dimensional shapes.
There have also been proposed, in Japanese Examined Patent Publication No. 7-323, for example, photocuring sheets comprising a laminate of a sheet base and a photocuring resin layer formed using a resin composition containing an acrylic resin, a compound with a reactive vinyl group and a photopolymerization initiator.
According to this process, however, since the sheet prior to photocuring contains a low molecular weight compound with a reactive vinyl group, the surface is tacky and the tackiness of the surface alters with time, such that its storage stability in the form of a roll is poor. Specifically, this causes problems in that, unless it is stored at low temperature, adhesion occurs and prevents unwinding, and the resin oozes out from both edges. The tackiness has also resulted in inconvenience during the printing step when the sheet is used as a printing sheet.
Japanese Unexamined Patent Publication No. 2-289611 discloses a resin composition comprising an acrylic resin containing an alicyclic epoxy group, and a cationic photopolymerization initiator; however, when this resin composition is used alone to form a sheet, an acrylic resin with a low glass transition temperature tends to cause clinging of the sheet to the die when it is used as an insert molding sheet. On the other hand, when an acrylic resin with a high glass transition temperature is used, the resulting fragility of the sheet presents the problem of poorer handleability.
It is an object of the present invention to provide a photocuring laminated sheet with excellent workability and storage stability and a lack of tackiness, that may be used in the manufacture of molded articles with satisfactory decorative designs and that gives molded articles with excellent mar-proof properties, weather resistance and chemical resistance.
In order to achieve this object, the invention provides a photocuring sheet wherein a photocuring resin composition (A) comprising an acrylic resin with a photopolymerizable functional group on the side chains (a-1) and a photopolymerization initiator (a-2), is laminated on a transparent base sheet (B), as well as photocuring decorative sheets and photocuring insert molding sheets employing it, molded articles employing these and a process for their manufacture.
Preferred embodiments of the invention will now be explained.
The photocuring sheet of the invention has a photocuring resin composition (A) comprising an acrylic resin with a photopolymerizable functional group on the side chains (a-1) and a photopolymerization initiator (a-2), laminated on a transparent base sheet (B). The structure with a photopolymerizable functional group on the polymer side chains promotes crosslinking reaction between the polymer side chains, so that it is not necessary to include a low molecular weight compound with a reactive vinyl group. This offers the advantage of giving a sheet with no tackiness and excellent storage stability.
The photopolymerizable functional group may be any one whose polymerization is promoted by light irradiation, and as a preferred group there may be mentioned the alicyclic epoxy group represented by the following structural formula (1). 
The compound having an alicyclic epoxy group on the side chain is not particularly restricted so long as it can undergo copolymerization with other (meth)acrylates, and compounds represented by the following structural formula (2) may be specifically mentioned. 
wherein R represents methyl or hydrogen, and n is an integer of 0-5.
The copolymerization of the compound with a photopolymerizable functional group on the side chain is preferably to 1-100 parts by weight per 100 parts by weight of the acrylic resin (a-1). At less than 1 part by weight, sufficient crosslinking may not occur within the polymer and the desired curing properties may not be achieved. In order to obtain a photocuring sheet with excellent mar-proof properties, the copolymerization of the compound with a photopolymerizable functional group on the side chain is even more preferably to 50-100 parts by weight.
By introducing into the acrylic resin a functional group that participates in crosslinking, it is possible to improve the curing property efficiently with a lower degree of crosslinking.
Known vinyl polymerizable monomers suitable for radical polymerization may also be copolymerized with the acrylic resin (a-1), if necessary. As examples of such vinyl polymerizable monomers there may be mentioned (meth)acrylates such as methyl (meth)acrylate, tricyclodecanyl (meth)acrylate and isobornyl (meth)acrylate; imide derivatives such as N-phenylmaleimide, cyclohexyl maleimide and N-butylmaleimide; hydroxyalkyl group-containing (meth)acrylates such as 2-hydroxyethyl (meth)acrylate and 2-hydroxypropyl (meth)acrylate; amide group-containing vinyl monomers such as acrylamide, methacrylamide and acrylonitrile; epoxy group-containing vinyl monomers such as allyl glycidyl ether and glycidyl (meth)acrylate; and olefinic monomers such as butadiene.
The acrylic resin (a-1) preferably has a glass transition temperature adjusted to 40-150xc2x0 C. When the glass transition temperature is lower than 40xc2x0 C., the die release properties of the sheet during insert molding may be inferior. When the glass transition temperature is higher than 150xc2x0 C., the sheet properties may be rendered fragile, thus impairing the handleability.
Considering the glass transition temperature of the resulting acrylic resin copolymer, it is preferred to use a vinyl polymerizable monomer with a high glass transition temperature of the homopolymer.
The molecular weight of the acrylic resin (a-1) is preferably in the range of 10,000-1,000,000. When the molecular weight is less than 10,000 the sheet may cling to the molding die due to the preheating for insert molding. On the other hand, a molecular weight of greater than 1,000,000 may be an obstacle to synthesis, or may render the acrylic resin less soluble with thorough stirring in organic solvents when the photocuring sheet is manufactured by a solvent casting method, and may result in an impaired outer appearance.
The process for production of the acrylic resin (a-1) is not particularly restricted, and any known radical polymerization process may be applied.
Using the acrylic resin (a-1) can give a photocuring sheet with-satisfactory storage stability in roll form without tackiness on the surface of the photocuring resin composition or without alteration of the tackiness of the surface with the passage of time.
The photopolymerization initiator (a-2) used for the invention may be a radical photopolymerization initiator that produces radicals upon light irradiation or a cationic photopolymerization initiator that produces an acid; a cationic photopolymerization initiator is used when the photopolymerizable functional group on the side chain is an alicyclic epoxy group.
The radical photopolymerization initiator is not particularly restricted so long as it is a known compound, but from the standpoint of yellowing in the curing process and deterioration upon exposure to weather, an initiator containing no amino groups in the molecule, such as an acetophenone-based or benzophenone-based initiator, may preferably be used. Preferred examples include 1-(4-dodecylphenyl)-2-hydroxy-2-methylpropan-1-one, 1-hydroxycyclohexylphenyl ketone, 2-hydroxy-2-methyl-1-phenylpropan-1-one and 1-(4-isopropylphenyl)-2-hydroxy-2-methylpropan-1-one. Care must be taken with some of these initiators since the temperature can temporarily exceed their boiling point depend on the molding process used. For increased surface hardness of the molded article, an oxygen poly merization inhibitor/curing inhibitor such as n-methyldiethanolamine may be added.
In addition to these photopolymerization initiators, various peroxides may also be added for curing by the heat of molding. When a peroxide is added to the photopolymerizable sheet, it must be cured at 150xc2x0 C. for about 30 seconds, and therefore it is preferred to use a peroxide with a low critical temperature, such as lauroyl peroxide, t-butylperoxy-2-ethyl hexanoate or 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane.
The amount of the radical photopolymerization initiator is preferably no greater than 5 wt % based on the weight of the compound with the photopolymerizable functional group in the side chain, because its residual amount after curing will affect the weather resistance, and the amount of the amino-based radical photopolymerization initiator, which is associated with yellowing in the curing process, is preferably no greater than 1 wt %.
The cationic photopolymerization initiator is not particularly restricted so long as it is a known compound, and specifically there may be mentioned diallyliodonium salts, triallylsulfonium salts and iron-arene (aromatic hydrocarbon) complexes. Preferred among these, from the standpoint of reactivity with the alicyclic epoxy (meth)acrylate and coloration, are triallylsulfonium salts represented by the following structural formula (3). 
wherein R1 represents a substituted or unsubstituted aromatic ring via a carbon/carbon bond or a carbon/sulfur bond, and R2 and R3 each represent a substituted or unsubstituted aromatic ring.
Selection of the cationic photopolymerization system offers advantages over a radical photopolymerization system, such as lower hardening shrinkage and more excellent cohesion with the resin, and therefore resistance to cracking and peeling of the cured layer, lower-odor air before and after curing, and no polymerization inhibition by oxygen.
The amount of the cationic photopolymerization initiator (a-2) to be added is preferably 0.1-10 parts by weight per 100 parts by weight of the acrylic resin.
The acrylic rubber elastomer described in Japanese Unexamined Patent Publication No. 9-3288 may be used as the acrylic rubber elastomer (a-3) of which at least the outermost layer is a hard resin layer. For example, it may be a graft copolymer wherein the resin composition includes a rubber layer of which the main constituent unit is an alkyl acrylate ester where the alkyl group has 1-8 carbon atoms, and the outer shell of the rubber layer is covered with a hard resin layer composed mainly of methyl methacrylate. The rubber layer is an elastomer composed mainly of an alkyl acrylate and has a glass transition temperature of no higher than 25xc2x0 C., and either the same substance may be used for the interior as for the outermost layer, or a hard resin layer with a different composition, composed mainly of methyl methacrylate, may be present.
The hard resin layer has a glass transition temperature of 25xc2x0 C. or higher, and it is preferably composed of a monomer comprising 40-99.5 wt % of methyl methacrylate, 49.5-0 wt % of an alkyl acrylate wherein the alkyl group has 1-8 carbon atoms, and 0.5-20 wt % of a monofunctional or polyfunctional vinyl monomer or vinylidene monomer that is copolymerized with these monomers.
The alkyl acrylate ester wherein the alkyl group has 1-8 carbon atoms is not particularly restricted, but methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate and the like are preferred. There are also no particular restrictions on-the copolymerizable monofunctional or polyfunctional vinyl or vinylidene monomer, but preferred monofunctional monomers include aromatic vinyls such as styrene, cyanated vinyls such as acrylonitrile and maleic acid derivatives such as cyclomaleimide.
Preferred polyfunctional monomers include difunctional monomers such as ethylene dimethacrylate, allyl methacrylate, allyl cinnamate, allyl sorbate and diallyl phthalate; trifunctional monomers such as triallyl cyanurate, triallyl isocyanurate, triallyl trimellitate, trimethylolpropane triacrylate, triallyl fumarate and triallyl maleate; and tetrafunctional monomers such as pentaerythritol tetraacrylate and pyromellitic tetraacrylate, any of which may be used alone or in combination.
To obtain the hard resin layer, a thiol compound such as octylmercaptane may be added as a polymerization chain transfer agent, if necessary, to control the molecular weight.
The acrylic rubber elastomer (a-3) which, may be used is preferably produced by emulsion polymerization. The polymerization temperature will generally be 50-160xc2x0 C., although this will differ slightly depending on the polymerization method.
The amount of the acrylic rubber elastomer (a-3) to be added is preferably in the range of 0.01-20 parts by weight per 100 parts by weight of the acrylic resin (a-1). When the acrylic, rubber elastomer is added at less than 0.01 part by weight, the effect of the rubber addition may not be achieved, and the film strength may be reduced resulting in a brittle film, if the glass transition temperature of the acrylic; resin (a-1) is above 100xc2x0 C. When the amount exceeds 20 parts by weight, the film properties after curing may be impaired.
The particle size of the acrylic rubber elastomer (a-3) is preferably 0.1-0.4 xcexcm. When the particle size is smaller than 0.1 xcexcm, the effect of the rubber addition may not be achieved and the film strength may be reduced if the glass transition temperature of the acrylic resin (a-1) is above 100xc2x0 C., and when the particle size is larger than 0.4 xcexcm the polymerization stability during polymer production may be poor, sometimes to an extent that the polymer to be produced by the invention cannot be obtained, or if it is obtained, the photocuring sheet may exhibit an inferior outer appearance.
A polyfunctional acrylate (a-4) which may be used for the invention must be solid at ordinary temperature. If it is not solid at ordinary temperature, tackiness will be produced in the sheet prior to curing, thus complicating the unwinding of the sheet.
The polyfunctional acrylate (a-4) which is solid at ordinary temperature is preferably one with a large number of double bonds per molecular weight unit, from the standpoint of the curing property. From the standpoint of weather resistance, however, it is preferably an aliphatic one with a small number of double bonds per molecular weight unit. So long as it is solid at ordinary temperature, it may even be an oligomer. In this case, a molecular weight exceeding 5000 may impair the curing property, and therefore the molecular weight of the oligomer is preferably less than 5000. As specific examples of acrylates that are solid at ordinary temperature there may be mentioned polyoxyethyleneglycol dimethacrylate, isocyanuric triacrylate, polyester acrylate, urethane acrylate, epoxy acrylate, and the like.
The polyfunctional acrylate (a-4) which is solid at ordinary temperature is preferably added to an amount in a range of 10-100 parts by weight per 100 parts by weight of the acrylic resin (a-1). When the polyfunctional acrylate which is solid at ordinary temperature is added at less than 10 parts by weight, the crosslinking density of the photocured sheet may approximately be the same as if no polyfunctional acrylate were added, and therefore no effect of addition may be achieved. When it is added at greater than 100 parts by weight, a large amount of the unreacted polyfunctional acrylate may remain in the sheet even after photocuring, resulting in inferior weather resistance.
A photocuring resin composition (A) used for the invention comprises [2] an acrylic resin with a photopolymerizable functional group on the side chains (a-1) and a photopolymerization initiator (a-2), [2] an acrylic resin with a photopolymerizable functional group on the side chains (a-1), a photopolymerization initiator (a-2) and an acrylic rubber elastomer (a-3) of which at least the outermost layer is a hard resin layer, [3] an acrylic resin with a photopolymerizable functional group on the side chains (a-1), a photopolymerization initiator (a-2) and a polyfunctional acrylate (a-4) which is solid at ordinary temperature, or [4] an acrylic resin with a photopolymerizable functional group on the side chains (a-1), a photopolymerization initiator (a-2), an acrylic rubber elastomer (a-3) of which at least the outermost layer is a hard resin layer and a polyfunctional acrylate (a-4) which is solid at ordinary temperature.
The transparent base sheet (B) may be selected as appropriate for the method of use and, for example, there may be mentioned ABS (acrylonitrile-butadiene-styrene copolymer) sheets, vinyl chloride-based resin sheets, polyolefin sheets such as polystyrene and polypropylene sheets, fluorine-based resin sheets, polyester sheets, polycarbonate sheets, soft acrylic sheets and the like. Preferred among these are thermoplastic resin sheets with an elongation of 100% or greater upon heating at 100xc2x0 C., because of their satisfactory-die-shape-following property in insert molding. From the standpoint of cohesion with the photocuring resin composition (A), weather resistance, transparency, etc., a thermoplastic acrylic resin sheet with a crosslinked rubber component is even more preferred.
Thermoplastic acrylic resin sheets with crosslinked rubber components include thermoplastic acrylic sheets obtained by extrusion molding of an acrylic resin with a multilayered structure such as that disclosed, for example, in Japanese Unexamined Patent Publication No. 9-263614.
The process for production of the photocuring sheet may be a process in which the photocuring resin composition (A) is thoroughly stirred and dissolved in an organic solvent or the like, coated onto the transparent base sheet (B) by a casting method using a knife coater, comma coater, reverse coater or the like, and drying for solvent removal to prepare a laminated sheet. When the resin solution is coated onto a base sheet made of a polyolefin such as polyethylene or polypropylene, it is preferred to [1] precoat the base sheet with a primer comprising a low molecular weight polyolefin or the like, or [2] activate the base sheet surface beforehand by corona discharge or the like (the corona discharge step is preferably carried out a short time before coating since the adherability is highest immediately after activation), in order to increase cohesion between the transparent base sheet and the photocuring resin composition.
The photocuring sheet of the invention may be provided with a printed layer on the transparent base sheet side to make a photocuring decorative sheet.
The following pigments may be used for the ink in the printed layer. As common pigments there may be used yellow pigments including azo-based pigments such as polyazo pigments, organic pigments such as isoindolinone, and inorganic pigments such as chrome yellow; red pigments including azo-based pigments such as polyazo pigments, organic pigments such as quinacridone, and inorganic pigments such as red iron oxide; blue pigments including organic pigments such as phthalocyanine blue and inorganic pigments such as cobalt blue; black pigments including organic pigments such as aniline black; and white pigments including inorganic pigments such as titanium dioxide.
The ink printing method used may be a known gravure rotary printing method or a screen printing method.
A photocuring resin composition according to the invention that employs no low molecular reactive vinyl compound and has crosslinking between the polymers exhibits no tackiness on the surface, results in few troubles during printing and gives satisfactory yields.
A photocuring sheet according to the invention may be fabricated as a photocuring insert molding sheet by forming in order a printed layer, an adhesive layer and if necessary a primer sheet on the transparent base sheet. In this case, the photocuring insert molding sheet preferably has a thickness in the range of 30-750 xcexcm. When the sheet thickness is less than 30 xcexcm, the sheet thickness on curved surface sections may be considerably reduced during deep draw molding, possibly resulting in inferior mar-proofness and poor chemical resistance properties in the obtained molded article. When the sheet thickness exceeds 750 xcexcm, the die shape-following property may be poor.
The adhesive layer may be selected from among any desired synthetic resin materials so long as they have the property of increasing cohesion between the printed layer and the molding resin or between the printed layer and the primer sheet. For example, when the molding resin is a polyolefin, the adhesive layer preferably contains a chlorinated polyolefin.
The primer sheet is formed when necessary, and it is made from a material that is compatible with the molding resin, in order to increase its cohesion with the molding resin. Generally speaking, it is preferably made from the same polymer material as the molding resin. A primer sheet also offers the advantage of minimizing transfer of surface defects of the injection molded article onto the photocuring resin composition. In such a case, the primer sheet must have a thickness that can absorb the surface defects of the molding resin while exhibiting the completely smooth upper surface of the photocuring resin composition.
A molded article fabricated using this photocuring insert molding sheet will now be explained.
The sheet is positioned with the photocuring resin composition (A) facing the die, the photocuring insert molding sheet is heated if necessary to allow the sheet to follow the shape of the die, and then the die is closed and the molten molding resin is injected for molding.
Finally, the molded article is taken out of the die and is irradiated with light using an ultraviolet irradiation lamp or the like. The irradiation dose is usually about 500-1000mJ/cm2. The light irradiation hardens the photocuring resin composition, and can give insert molded articles with hard coatings formed on the surface.
When a molded article decorated with a photocuring insert molded sheet of the invention is to be used primarily outdoors, an ultraviolet absorber or light stabilizer may be added to the sheet. As ultraviolet absorbers there may be used organic substances such as benzotriazole, benzophenone and salicylic acid esters, or inorganic substances such as zinc oxide, cerium oxide and titanium oxide in fine particle form with a size of no greater than 0.2 xcexcm. As light stabilizers there may be used radical scavengers including hindered amine-based radical scavengers and piperidine-based radical scavengers, such as bis-(2,2,6,6-tetramethyl-4-piperidinyl) sebacate.
The molded article obtained in this manner is decorated with color and a design during the molding, and a short period of light irradiation can improve the mar-proof property, chemical resistance and weather resistance. According to the invention, therefore, it is possible to shorten the process, achieve improved yields and reduce effects on the environment, compared to the current means of spray painting after molding.
The photocuring sheet of the invention can also be used as a laminate on an injection molded article, by lamination on an already injection molded resin either directly or via an adhesive layer.